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  • Avoiding explicit recursion in Haskell

    - by Travis Brown
    The following simple function applies a given monadic function iteratively until it hits a Nothing, at which point it returns the last non-Nothing value. It does what I need, and I understand how it works. lastJustM :: (Monad m) => (a -> m (Maybe a)) -> a -> m a lastJustM g x = g x >>= maybe (return x) (lastJustM g) As part of my self-education in Haskell I'm trying to avoid explicit recursion (or at least understand how to) whenever I can. It seems like there should be a simple non-explicitly recursive solution in this case, but I'm having trouble figuring it out. I don't want something like a monadic version of takeWhile, since it could be expensive to collect all the pre-Nothing values, and I don't care about them anyway. I checked Hoogle for the signature and nothing shows up. The m (Maybe a) bit makes me think a monad transformer might be useful here, but I don't really have the intuitions I'd need to come up with the details (yet). It's probably either embarrassingly easy to do this or embarrassingly easy to see why it can't or shouldn't be done, but this wouldn't be the first time I've used self-embarrassment as a pedagogical strategy. Background: Here's a simplified working example for context: suppose we're interested in random walks in the unit square, but we only care about points of exit. We have the following step function: randomStep :: (Floating a, Ord a, Random a) => a -> (a, a) -> State StdGen (Maybe (a, a)) randomStep s (x, y) = do (a, gen') <- randomR (0, 2 * pi) <$> get put gen' let (x', y') = (x + s * cos a, y + s * sin a) if x' < 0 || x' > 1 || y' < 0 || y' > 1 then return Nothing else return $ Just (x', y') Something like evalState (lastJustM (randomStep 0.01) (0.5, 0.5)) <$> newStdGen will give us a new data point.

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  • Duplicate Name Exists error from virtual PC with NAT network configuration

    - by Phred
    Whenever I change the network configuration to NAT for a virtual machine running under Virtual PC 2007, I get "A duplicate name exists on the network." error. There is no other machine on my network with this name and running the VM in any other network configuration doesn't cause this error. This seems to be a common problem with Virtual PC 2007 based on a google search but no-one seems to have a solution to it. So far, I've discovered that turning off NETBIOS over TCP causes the problem to go away but I need to join this VM to a domain and you can't do that if NETBIOS over TCP is turned off.

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  • Problems with validates_inclusion_of, acts_as_tree and rspec

    - by Jens Fahnenbruck
    I have problems to get rspec running properly to test validates_inclusion_of my migration looks like this: class CreateCategories < ActiveRecord::Migration def self.up create_table :categories do |t| t.string :name t.integer :parent_id t.timestamps end end def self.down drop_table :categories end end my model looks like this: class Category < ActiveRecord::Base acts_as_tree validates_presence_of :name validates_uniqueness_of :name validates_inclusion_of :parent_id, :in => Category.all.map(&:id), :unless => Proc.new { |c| c.parent_id.blank? } end my factories: Factory.define :category do |c| c.name "Category One" end Factory.define :category_2, :class => Category do |c| c.name "Category Two" end my model spec looks like this: require 'spec_helper' describe Category do before(:each) do @valid_attributes = { :name => "Category" } end it "should create a new instance given valid attributes" do Category.create!(@valid_attributes) end it "should have a name and it shouldn't be empty" do c = Category.new :name => nil c.should be_invalid c.name = "" c.should be_invalid end it "should not create a duplicate names" do Category.create!(@valid_attributes) Category.new(@valid_attributes).should be_invalid end it "should not save with invalid parent" do parent = Factory(:category) child = Category.new @valid_attributes child.parent_id = parent.id + 100 child.should be_invalid end it "should save with valid parent" do child = Factory.build(:category_2) child.parent = Factory(:category) # FIXME: make it pass, it works on cosole, but I don't know why the test is failing child.should be_valid end end I get the following error: 'Category should save with valid parent' FAILED Expected #<Category id: nil, name: "Category Two", parent_id: 5, created_at: nil, updated_at: nil to be valid, but it was not Errors: Parent is missing On console everything seems to be fine and work as expected: c1 = Category.new :name => "Parent Category" c1.valid? #=> true c1.save #=> true c1.id #=> 1 c2 = Category.new :name => "Child Category" c2.valid? #=> true c2.parent_id = 100 c2.valid? #=> false c2.parent_id = 1 c2.valid? #=> true I'm running rails 2.3.5, rspec 1.3.0 and rspec-rails 1.3.2 Anybody, any idea?

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  • Recommend me a good book/source for learning for Web Design

    - by Idlecool
    Hi, While web development/design, I found coding it self is not enough to develop an attractive website so i have urge to learn photoshop.. i want to know what can be the best way to learn photoshop (preferably Photoshop CS4 Extended) mainly for website design. I experienced web tutorials are not enough for effectively learning computer languages, but not sure about photoshop coz it is a different job.. please recommend me a book / web tutorial for learning Photoshop.

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  • NSUrlconnection problem receiving data from some filehosts

    - by Tammo
    hello again, i am trying to develop an downloadmanager. i can now download files from almost anywhere on linkclick. in the - (BOOL)webView:(UIWebView*)webView shouldStartLoadWithRequest:(NSURLRequest*)request navigationType:(UIWebViewNavigationType)navigationType i check if the url is a url to a binaryfile like a zipfile. than i setup a nsurlconnection NSMutableURLRequest *urlRequest = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalCacheData timeoutInterval:20.0]; [urlRequest setValue:@"User-Agent" forHTTPHeaderField:@"Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en) AppleWebKit/418.9 (KHTML, like Gecko) Safari/419.3"]; NSURLConnection *mainConnection = [NSURLConnection connectionWithRequest:urlRequest delegate:self]; if (nil == mainConnection) { NSLog(@"Could not create the NSURLConnection object"); } (void)connection:(NSURLConnection )connection didReceiveResponse:(NSURLResponse)response { self.tabBarController.selectedIndex=1; [receivedData setLength:0]; percent = 0; localFilename = [[[url2 absoluteString] lastPathComponent] copy]; NSLog(localFilename); NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES); NSString *documentsDirectory = [paths objectAtIndex:0] ; NSString *appFile = [documentsDirectory stringByAppendingPathComponent:localFilename]; [[NSFileManager defaultManager] createFileAtPath:appFile contents:nil attributes:nil]; [downloadname setHidden:NO]; [downloadname setText:localFilename]; expectedBytes = [response expectedContentLength]; exp = [response expectedContentLength]; NSLog(@"content-length: %lli Bytes", expectedBytes); file = [[NSFileHandle fileHandleForUpdatingAtPath:appFile] retain]; if (file) { [file seekToEndOfFile]; } } (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data { if (file) { [file seekToEndOfFile]; } [file writeData:data]; [receivedData appendData:data]; long long resourceLength = [receivedData length]; float res = [receivedData length]; percent = res/exp; [progress setHidden:NO]; [progress setProgress:percent]; NSLog(@"Remaining: %lli KB", (expectedBytes-resourceLength)/1024); [kbleft setHidden:NO]; [kbleft setText:[NSString stringWithFormat:@"%lli / %lli KB", expectedBytes/1024 ,(resourceLength)/1024]]; } in the connectiondidfinish loading i close the file. all working fine for nearly every hoster except hosters wich have a capture procedure before like filedude.com in the uiwebview i can surf to the downloadpage enter the captcha and get the downloadlink. when i click on it the file will be created in the documentsdir with the filename and the download starts but he dont get any data. every file has 0kb and the NSLog(@"content-length: %lli Bytes", expectedBytes); gives out something like 100-400 byte . can somebody help me solve this problem? kind regards

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  • Singleton with eager initialization

    - by jesper
    I have class X that takes much time to initialize itself. I want to make that class singleton and force its creation when rails application starts. I've made the singleton: class X @@instance = nil def self.instance if @@instance.nil? @@instance = X.new puts 'CREATING' end return @@instance end private_class_method :new end The problem is that every time I use this class I see 'CREATING' in logs. I've tried to put creation of class in initializers directory but it doesn't work either.

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  • Android , Read in binary data and write it to file

    - by Shpongle
    Hi all , Im trying to read in image file from a server , with the code below . It keeps going into the exception. I know the correct number of bytes are being sent as I print them out when received. Im sending the image file from python like so #open the image file and read it into an object imgfile = open (marked_image, 'rb') obj = imgfile.read() #get the no of bytes in the image and convert it to a string bytes = str(len(obj)) #send the number of bytes self.conn.send( bytes + '\n') if self.conn.sendall(obj) == None: imgfile.flush() imgfile.close() print 'Image Sent' else: print 'Error' Here is the android part , this is where I'm having the problem. Any suggestions on the best way to go about receiving the image and writing it to a file ? //read the number of bytes in the image String noOfBytes = in.readLine(); Toast.makeText(this, noOfBytes, 5).show(); byte bytes [] = new byte [Integer.parseInt(noOfBytes)]; //create a file to store the retrieved image File photo = new File(Environment.getExternalStorageDirectory(), "PostKey.jpg"); DataInputStream dis = new DataInputStream(link.getInputStream()); try{ os =new FileOutputStream(photo); byte buf[]=new byte[1024]; int len; while((len=dis.read(buf))>0) os.write(buf,0,len); Toast.makeText(this, "File recieved", 5).show(); os.close(); dis.close(); }catch(IOException e){ Toast.makeText(this, "An IO Error Occured", 5).show(); } EDIT: I still cant seem to get it working. I have been at it since and the result of all my efforts have either resulted in a file that is not the full size or else the app crashing. I know the file is not corrupt before sending server side. As far as I can tell its definitely sending too as the send all method in python sends all or throws an exception in the event of an error and so far it has never thrown an exception. So the client side is messed up . I have to send the file from the server so I cant use the suggestion suggested by Brian .

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  • problem with presentModalViewController

    - by pbcoder
    If my iPad is in Landscape mode and presentModalViewController is called the view automatically turns into portrait mode. Any solutions? UIViewController * start = [[UIViewController alloc]initWithNibName:@"SecondView" bundle:nil]; start.modalTransitionStyle = UIModalTransitionStyleFlipHorizontal; start.modalPresentationStyle = UIModalPresentationFormSheet; [self presentModalViewController:start animated:YES]; In SecondView I've already added: - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation { return YES; }

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  • Trouble in type casting or any ideas ???

    - by ahmet732
    I write code below but it gives warning.. I identified dbdesc as a NSString but nothing changes.. How will I handle with this type of warnings? **[self.searchDetailViewController setText:appDelegate.dbdesc];** warning:'searchDetailViewController' may not respond to 'setText:' (Messages without a message method signatures will be assumed to return (id) and accept '...'as arguments )

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  • How to put fear of God (law) into Wi-Fi hacking neighbors [closed]

    - by Shakehar
    I live in an apartment and some new guys have apparently moved into one of the apartments. They have been shamelessly hacking into my WiFi. Mine was initially a WEP encrypted network and out of laziness I just limited and reserved the IPS on my router for the people in my house. Yesterday I had to free up an IP for a guest in my house but before he could join the network these guys connected in. I have changed my encryption to WPA2 and hope they dont have the hardware/patience required to hack into it, but there are many wi-fi networks in my apartment most of which are secured using WEP. I don't really want to call the police on them. Is there any way to deter them from misusing other people's wi-fi ? I have gone through I think someone else has access to my wireless network. What next? but I have already taken the steps mentioned there.

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  • Faking a UITableView selection

    - by John Smith
    I have a UITableView where a selection displays some cool stuff in another part of the display. I tried to fake a keypress by writing: NSIndexPath *ip = [NSIndexPath indexPathForRow: 1 inSection:0]; [tableView selectRowAtIndexPath:ip animated:NO scrollPosition:UITableViewScrollPositionTop ]; [self tableView:tableView didSelectRowAtIndexPath:ip]; However it seems the last line is not being run. How can I make it run?

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  • simple question of pushViewController

    - by user217572
    messagecontroller is nothing but object of initialize nib file. [self.navigationController pushViewController:messageController animated:YES]; this statement executes in normal condition this statement also works on state maintainace testing , this line executes properly but not open new view ,why?

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  • Re-define File::dirname ruby method

    - by jrhicks
    I'm trying to redefine the File.dirname method to first change %20s to spaces. But the following gives me an error class File old_dirname = instance_method(:dirname) define_method(:dirname) { |s| s = s.gsub("%20"," ") old_dirname.bind(self).call(s) } end This trhows a NameError exception: undefined method 'dirname' for class 'File' What is the right way to do this?

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  • Creating an Expando object in Ruby

    - by tyndall
    Is there a better way to write this Expando class? The way it is written does not work. I'm using Ruby 1.8.7 starting code quoted from https://gist.github.com/300462/3fdf51800768f2c7089a53726384350c890bc7c3 class Expando def method_missing(method_id, *arguments) if match = method_id.id2name.match(/(\w*)(\s*)(=)(\s*)(\.*)/) puts match[1].to_sym # think this was supposed to be commented self.class.class_eval{ attr_accessor match[1].to_sym } instance_variable_set("#{match[1]}", match[5]) else super.method_missing(method_id, *arguments) end end end person = Expando.new person.name = "Michael" person.surname = "Erasmus" person.age = 29

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  • iOS dynamic object creation

    - by Abdul Ahmad
    I've worked with Xcode and iOS on a few personal projects and have always used non-object-oriented designs for everything... just because I've been doing mostly learning/experimenting with things. Now I've been trying to implement some object oriented design into one game I've made previously. The idea is, I have a space ship that can shoot bullets. In the past I basically added the UIImageView to the storyboard and then connected it to the .h file and from there did things on it like move it around or whatever (using CGPointMake). The idea now is to make a method (and a whole other class soon) that will create a UIImageView programmatically, allocate it, add it to the superview etc... I've got this working so far, easy stuff, but the next part is a bit harder. Where do I store this local variable "bullet"? I've been saving it to an NSMutableArray and doing the following: (actually here are the methods that I have) -(void)movement { for (int i = 0; i < [array1 count]; i ++) { UIImageView *a = [array1 objectAtIndex:i]; a.center = CGPointMake(a.center.x + 2, a.center.y); if (a.center.x > 500) { [array1 removeObjectAtIndex:i]; [a removeFromSuperview]; } } } -(void)getBullet { UIImageView *bullet = [[UIImageView alloc] initWithFrame:CGRectMake(ship.center.x + 20, ship.center.y - 2, 15, 3)]; bullet.image = [UIImage imageNamed:@"bullet2.png"]; bullet.hidden = NO; [self.view addSubview:bullet]; [array1 addObject:bullet]; } (by the way, array1 is declared in the .h file) and theres a timer that controls the movement method timer = [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:@selector(movement) userInfo:nil repeats:YES]; first question is: what is the correct way of doing this? Storing a bullet for example until it is removed from the superview, should I store it another way? and another question is, when I remove a UIImageView from the superview, does that remove it from memory so its not using up system resources? Thank you for the help! (will update if I Think of other questions

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  • Cutting up videos (excerpting) on Mac OS X -- iMovie produces super-large files

    - by markvgti
    I need to cut out parts of a video (+ the associated audio, of course) to make a short clip. For example, take 2 minutes from one location, 3 minutes from another part of the video, 30 seconds from another location and join it all together to form one single clip. The format of the input video is mp4 (H.264 encoding, AFAICR). Don't need very sophisticated merges or transitions from one part to the next, or sophisticated banners (text) on-screen, but some ability to do so would be a plus point. I've done this with iMovie in the past, but where the original file was under 5MB/min of play time, the chopped-up version was over 11MB/min of play time, which to me seems really bad. Is there a better/different way of doing this on OS X? Looking for free (gratis) solutions. OS: OS X 10.9.3

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  • EOF error using recv in python

    - by tipu
    I am doing this in my code, HOST = '192.168.1.3' PORT = 50007 s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.connect((HOST, PORT)) query_details = {"page" : page, "query" : query, "type" : type} s.send(str(query_details)) #data = eval(pickle.loads(s.recv(4096))) data = s.recv(16384) But I am continually getting EOF at the last line. The code I am sending with, self.request.send(pickle.dumps(results))

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  • Set background of UITableview in interface builder?

    - by Martin
    In iPhone I need to set the background view of a UITableview. From 3.2 SDK I can in code use something like: [self.tableView setBackgroundView:[[UIImageView alloc] initWithImage:[UIImage imageNamed:@"mycustombackground.png"]]]; But how can I do the same in interface builder? It would be great if I could just set the background view directly in interface builder. But can't seem to find this property, where is it? Thanks!

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